sharp slope of IF amplifier output improves video detector output..how?

Thread Starter

Himanshoo

Joined Apr 3, 2015
265
Hi,
The following is a part of IF section of a television receiver...

As it is being visible that the output response of the IF section in superheterodyne receiver has a sharp slope which is necessary for correction of output response of video detector. It has been said that this sharp slope is necessary so that it could provide an attenuation to the lower frequencies near the picture carrier so that they should not gets an extra amplification.

So my query is that how by modifying the output response of the IF section ( sharp slope) prevents lower frequency to gets amplified at the output of the video detector...??

Screen Shot 2018-03-13 at 8.19.29 pm.png
 

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crutschow

Joined Mar 14, 2008
30,755
Filters are used to suppress undesired frequencies outside the frequency of interest since those can cause interference or distortion of the signal.
So the steeper the slope of the filter (gain roll-off is faster) the more the undesired frequencies are rejected.
The ideal filter (not achievable in practice) would have vertical edges so that anything outside the frequencies of interest would be completely suppressed.
 

Thread Starter

Himanshoo

Joined Apr 3, 2015
265
well I am talking in context of television reception ...

the input to the video detector is taken from the IF section..now this sharp slope is meant for vestegial side band correction.
Without this correction the output of the video detector is like below...

non correction.png

As it is clear from the figure that the frequencies near the picture carrier gets a double amplification which is not required...a corrected output response will make the output response to all frequencies to a unity..

so i want to know that exactly how does a sharp sloped IF curve will give a response like this..

Screen Shot 2018-03-13 at 8.22.33 pm.png
 
Last edited:

crutschow

Joined Mar 14, 2008
30,755
The signal has two sidebands around the picture carrier (one from the vestigial side) but the one from the vestigial side is not wanted in the demodulated video.
Since the lower frequency portion of those sidebands are close together it requires a sharp (fast) rolloff of the filter to suppress the undesired sideband without significantly attenuating the desired sideband.
 

BR-549

Joined Sep 22, 2013
4,928
It's because a little bit of the video carrier.....of the UN-wanted sideband is transmitted.

It's an asymmetrical AM transmission.


A vestigial.....meaning a small useless part.......of the UN-wanted sideband is transmitted.
 

MisterBill2

Joined Jan 23, 2018
13,090
@crutschow
how would the response of the video detector look like if the IF section curve will be somewhat broader rather that sharp....??
The reason that the picture looks better when the IF passband has a sharper slope is that it prevents other frequencies from delivering interference to the video signal. Eliminating both noise and the audio signal removes the unwanted interference, thus maaking the picture better.Unfortunately here in the USA we now have digital transmissions and so the detection process is far more complex than ever before.
 

Thread Starter

Himanshoo

Joined Apr 3, 2015
265
Ok...one more thing to know is that...why the lower frequencies experience a double amplification at the output of video detector..?
 

Thread Starter

Himanshoo

Joined Apr 3, 2015
265
The reason that the picture looks better when the IF passband has a sharper slope is that it prevents other frequencies from delivering interference to the video signal
But don't u think that having a sharper IF slope will neglect important low frequencies as somewhere in previous post crutschow have said that low frequency portions are closely spaced..
 

crutschow

Joined Mar 14, 2008
30,755
Ok...one more thing to know is that...why the lower frequencies experience a double amplification at the output of video detector..?
having a sharper IF slope will neglect important low frequencies as somewhere in previous post crutschow have said that low frequency portions are closely spaced..
AM modulation (which the video is) produces sum and difference frequencies around the carrier frequency. The low modulation frequencies are thus closest to the carrier frequency.
To generate the vestigial signal most of the lower sideband up to within about 0.75MHz of the carrier is filtered out, leaving a double side-band signal up to that point.
Since those lower sidebands aren't wanted in the demodulated signal, they are filtered by the receiver as much as practical, while still leaving the lower frequency upper sidebands.

Read this for more info on the vestigial modulation scheme used for NTSC.
 
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